It has long been observed that cathode ray tube displays are relatively unusable in high ambient light environments. This problem has prevented the use of cathode ray tube displays in aircraft, such as jet fighter aircraft, in which high ambient light conditions are normal during daylight operation. To overcome this problem, direction sensitive contrast enhancement filters have been developed which are mounted on the face plate of cathode ray tubes in this application. These filters prevent most ambient light from striking the face of the tube but allow an observer to see a usable visual display on the face of the cathode ray tube under the high ambient light conditions. These contrast enhancement filters typically comprise a plate having a large number of very small holes therethrough, and the plate and the interior areas of the holes are all blackened to absorb ambient light before it strikes the fact of the cathode ray tube. These filters allow an observer to see the display on the face of the tube within a cone of vision in front of the cathode ray tube. The size of this cone of vision is a function of the ratio of the hole diameter to the filter thickness. In the prior art, direction sensitive contrast enhancement filters were unusable with shadow mask type color cathode ray tubes due to moire interference patterns which were created. These patterns are created because the amount of light that is transmitted by each phosphor dot depends upon the open area of the holes in the filter that expose each phoshor dot. This open area will in general be different for each phosphor dot and since both the phosphor dots and filter holes are each ordered arrays, a moire modulation pattern is created which interferes with the desired picture and/or data. It has been generally accepted by the industry that direction sensitive contrast enhancement filters are not useable with shadow mask tubes because of this moire interference. It is this problem in the prior art that the subject invention solves.
It is an object of this invention to teach a method for making a direction sensitive contrast enhancement filter for a specific color cathode ray tube having dots of color phosphor of any size and shape and in any ordered arrangement by selecting filter hole size and spacing to minimize moire interference patterns to unnoticeable or unobjectionable levels.